DRI is a solid granular material which is produced by reacting iron ores (mainly iron oxides in the form of lumps, concentrated pellets or mixtures thereof) with a reducing gas, composed principally of hydrogen and carbon monoxide, at a temperature in the range of 750 to 1100° C.
Typical DRI production facilities are disclosed for example in U.S. Pat. Nos. 3,779,741; 3,765,872; 4,150,972; 4,336,063; 4,834,792; and 5,078,787. Such systems commonly comprise vertical flow reactors having a reduction zone in the upper portion thereof wherein the hot reducing gas flows upwardly counter-current to a descending body of iron ore, and optionally a cooling zone in which the reduced ore (DRI) in the form of sponge iron is cooled with a cooling gas. Alternatively, the DRI is directly hot discharged from the reactor and fed to a DRI melting furnace or to a separate cooling vessel.
The reducing gas is generally obtained by reformation of natural gas in an external catalytic reformer (see for example U.S. Pat. Nos. 3,765,872 and 4,150,972) or, more advantageously, inside the reduction reactor by exploiting the DRI as an effective reformation catalyst (see U.S. Pat. Nos. 4,336,063, 4,668,284 and 5,110,350).
The external catalytic reformer comprises a bank of catalyst-filled tubes located in a heating chamber. Said tubes are externally heated by hot combustion products (including CO2 in significant amount) released by the burners and finally vented into the atmosphere via an exhaust stack.
The reducing gas, introduced into the reactor in the lower part of the reduction zone, is subsequently removed from the top of the reducing zone and divided in two streams: the majority is treated to be upgraded by eliminating most of the reduction reaction by-products (carbon dioxide and water), while the small remainder stream is purged sufficiently to prevent accumulation of inert gases (like N2) in the system and typically can be used as a heating fuel.
It has long been known in the art how to remove water and carbon dioxide to upgrade the spent reducing gas. In particular, U.S. Pat. Nos. 2,547,685, 4,001,010; 4,129,281; 3,853,538; and 4,046,557 teach the removal of the water by quench cooling and of the CO2 by chemical absorption in a unit where the CO2 containing gas is contacted with a liquid solution which reacts with said CO2, leading to a pure CO2 off-gas stream leaving the plant.
When an external catalytic reformer is used, the upgraded reducing gas stream, after being combined with the make up of reformed gas, is heated in a gas heater and finally recycled back into the reduction reactor wherein, as previously indicated, the reduction reaction takes place.
In a Zero-Reformer Plant, i.e. a plant without an external reformer, the upgraded reducing gas stream, now largely depleted of CO2, is finally fed to the reduction reactor after being saturated with hot water, which may be taken from the off gas cooler as suggested in U.S. Pat. No. 5,110,350. The water content in the recycle reducing gas stream promotes auto-reforming of the natural gas previously fed into the stream of the upgraded reducing gas. The mixture of natural gas, water and recycled gas is subsequently heated in a gas heater (typically assisted by an O2 injection to achieve a higher temperature) and fed into the reduction reactor wherein, as previously indicated, the reformation and reduction reactions simultaneously take place.
Alternatively, CO2 can be removed from a mixture of gases by using a physical adsorption system of the PSA or VPSA type (exemplary patents are U.S. Pat. Nos. 3,788,037; 4,869,894; 4,614,525; 5,026,406; 5,152,975; 5,833,734; 5,858,057 and 6,027,545) or by other means known in the industry.
U.S. Pat. No. 6,027,545 is the first to suggest applying this technology in a direct reduction plant. However, in the method disclosed by this patent, there is no selective CO2 removal by a chemical absorber system. Also, the PSA system is not used to separate the CO2 from the majority of the spent gas stream that is recycled, but instead is used to recover a high purity hydrogen stream from the relatively small amount of gas that is purged and subsequently recycles back the separated H2 so as to be added to and used as part of the recycled reducing gas (and not as a heater fuel gas).
U.S. Pat. No. 6,562,103 discloses a direct reduction process incorporating a PSA unit for the removal of carbon dioxide from the spent reducing gas. This patent however teaches only a particular way of purging the PSA units but does not teach nor suggest treating the tail gas 60 which will be burned in the heater 72 so that only hydrogen would be burned in the heater (to the substantial exclusion of the rest of carbon-containing gases, mainly CO and CH4). Consequently, the CO2 produced by burning the carbon-containing tail gas 60 and the natural gas 64 will be released uncontrolled to the atmosphere (and will not be selectively separated in a chemical CO2 removal plant).
Thus, in a typical direct reduction plant, the main emission sources of CO2 are located (1) in the absorber column of the CO2 removal plant (characterized as a selective CO2 emission) and (2) in the process gas heater stack (characterized as a non-selective CO2 emission). In addition, when an external catalytic reformer is used as the reducing make up gas source, an additional non-selective emission of CO2 will issue from the reformer stack.
As a consequence of the increasing concern about the greenhouse effect attributed to the increased presence of CO2 in the atmosphere, measures have to be considered to limit the consequences of this problem in the world. A first measure is essentially to reduce the CO2 emissions to the atmosphere. For this reason, DRI producers are facing the necessity to develop direct reduction processes where the CO2 emissions to the atmosphere are significantly decreased.
The objects of the invention are achieved by providing a method for the direct reduction of iron ores which comprises a chemical absorption system, to extract a stream of almost pure CO2 from the spent gas removed from the reactor, the heater, and the reformer resulting in use mainly of hydrogen as the fuel for the burners: in this way essentially a carbon free emission is released from the reformer and/or the heater stack.
The only carbon-containing fuel burned in the heater and/or the reformer, which involves the release of CO2 after combustion reactions therein, is a small amount of reducing gas; comprising CO, CO2 and CH4, necessarily removed from the system to purge inert elements (like nitrogen) which otherwise accumulate continuously, and, if needed, a minimum stream of natural gas required to produce a visible flame that allow safe monitoring of burner ignition.
Moreover, this invention suggests producing the hydrogen required as fuel from the reduction system itself. In particular, a physical adsorber system of the PSA type is used to recover hydrogen from a portion of the gas stream previously upgraded by the chemical CO2 absorber plant. Hydrogen separation may also be carried out by other means, for example by gas separation membranes; including optionally a combination of the PSA/VPSA and gas membrane systems. Furthermore, it is clear that neither the PSA/VPSA system and/or the gas membrane system are installed alone or in combination as an alternative of the chemical absorption system mentioned above, but is/are additional units, located offline of the process gas recycle circuit, whose aim is treating an offline portion of the process gas, to recover pure hydrogen for burner combustion and thus to permit rejection back to the process gas recycle circuit the other carbon-containing elements.
In this way, a large portion of the CO2 production from the heater and reformer burners (now mainly fed with hydrogen instead of carbon bearing fuels) is automatically diverted to the chemical absorption unit where almost all the CO2 will be withdrawn from the DRI reduction system in a contained manner as pure technical gas.
This invention can be usefully incorporated to a reduction system both with an external reformer and a Zero Reformer. Nevertheless, it is clear that a Zero Reformer system, where an external reformer is not required, is preferable, because the amount of hydrogen used as fuel has to be sufficient only for the heater burners.
Documents cited in this text (including the foregoing listed patents), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.